What makes you think they can't be used? (import the SmoothMover class and take a look at its code)

I just didn't notice any methods in the API that could support a floating point. I'm not sure how I didn't notice the SmoothMover class though, thanks! One more question though, are there any methods which accept a double as a valid parameter that can be used in the World or Actor classes?

No. All numeric parameters seem to be of int type. This is like asking "what is the point of furniture in a house if you don't use furniture to build the house." All the trigonometric, exponential and logarithmic methods in the java.lang.Math package use double values (for parameter and return types).